Fuel cells are classified into various types according to the types of electrolytes or the types of electrodes.
Typical types are alkaline type, phosphoric acid type, molten carbonate type, solid electrolyte type and solid polymer type. Of these, fuel cells of the solid polymer type capable of working at temperatures ranging from low temperatures (about −40° C.) to about 120° C. have been paid attention, and development and practical use of them as power sources for low-pollution automobiles have been promoted. As uses of the polymer electrolyte fuel cells, vehicle drive sources and stationary power sources have been studied. In order to apply the fuel cells to these uses, durability over a long period of time is required.
In this polymer solid fuel cell, a polymer solid electrolyte is sandwiched between an anode and a cathode. A fuel is fed to the anode, oxygen or air is fed to the cathode, and oxygen is reduced in the cathode to produce electricity. As the fuel, hydrogen or methanol is mainly used.
For increasing reaction rate in a fuel cell and thereby enhancing energy conversion efficiency of the fuel cell, a layer containing a catalyst (also referred to as a “fuel cell catalyst layer” hereinafter) has been provided on the surface of the cathode (air electrode) or the anode (fuel electrode) of the fuel cell in the past.
As the catalyst, a noble metal is generally used, and of such noble metals, platinum that is stable at a high electric potential and has high activity has been mainly used. However, since platinum is expensive and its resource quantity is limited, development of alternative catalysts has been demanded. Further, the noble metal used on the cathode surface is sometimes dissolved in an acidic atmosphere, and there is a problem that the noble metal is not suitable for the uses requiring long-term durability. On this account, development of catalysts that are not corroded in an acidic atmosphere, are excellent in durability, have high oxygen reduction activity and are low in the electric power generation cost has been strongly demanded.
On the other hand, carbon has been used in the past as a support for supporting the catalyst metal.
The catalytic activity of the carbon cannot be enhanced unless its specific surface area is increased, and therefore, particle diameters of the carbon need to be decreased. However, there is technical limitation on the decrease of the particle diameters of carbon, and satisfactory catalytic activity cannot be obtained yet.
Moreover, carbon has low heat resistance, and when reaction proceeds in a fuel cell, carbon is corroded and is lost. Hence, there occurs a phenomenon that the catalyst metal particles supported on the carbon are liberated from the support and the catalyst metal is aggregated. As a result, the effective area is decreased and the cell performance is lowered.
In a patent literature 1, a case of using specific carbon as a support for a fuel cell catalyst is disclosed. It is described that the fuel cell catalyst using this carbon (described also as “carbon powder” in the specification) as a support for supporting a catalyst of platinum or a platinum alloy has higher electric power generation efficiency and longer life than fuel cell catalysts using other carbons.
In the patent literature 1, however, platinum is essential as the catalyst metal, and it is difficult to obtain satisfactory electric power generation cost even if the above effects are taken into consideration.